1. Field of the Invention
The present invention relates generally to "smart" built-in-test techniques for automatic testing of electronic equipment, and specifically, the implementation of improved smart built-in-test techniques that monitor and correlate real-time fault and environment data for precise classification of fault behavior as OK, Intermittent, or HARD.
2. Description of the Prior Art
In aircraft avionics, high rates of Cannot Duplicates (CND's), i.e., failures in flight that cannot be verified on the ground, and Retest OK's (RTOK's), i.e., units which fail testing on the aircraft but test good in the maintenance shop, lead to excessive line and shop maintenance times, and maintenance equipment loading. This, in turn, results in decreased sortie rates, long logistic tails, and higher life cycle cost.
These problems have been addressed by the development of BIT (Built-in Test) technology whose techniques have led to the conclusion that intermittent faults are a major cause of many of the CND's and RTOK's.
Conventional avionics uses Built-in-Test (BIT) filters (software routines) to counter-act intermittent failures. However, BIT filters do not properly classify an intermittent fault as Intermittent. This may result in Fault Detection (FD) and Fault Isolation (FI) errors. More specifically, if an intermittent fault occurs, conventional BIT techniques declare an OK if the BIT filter does not trip, or HARD if it does trip. Most of the current effort to combat the CND and RTOK problems have been spent on improving Fault Detection (FD) and Fault Isolation (FI). While FD and FI are part of the solution, they are not the complete solution because intermittent failures are a major part of the problem. In addition, "band-aid fixes" that do address the intermittent failure problem often create problems of their own.
One of the most serious disadvantages of conventional BIT filters is the two-state behavior classification of Line Replaceable Units (LRUs), Weapons Replaceable Assemblies (WRAs) and Line Replaceable Modules (LRMs) as good (OK) or hard faulted (HARD). If a hard fault (HARD) is declared, conventional BIT uses hard fault diagnostic logic to isolate the fault even if the fault is intermittent, this results in fault isolation errors which cause CND and RTOK problems.
Thus, it would be highly desirable to implement a Smart BIT methodology that performs a three-state fault classification including OK, Intermittent, or HARD fault states. Knowing the true fault behavior that is occurring allows proper diagnostic procedures to be used for fault isolation and avoids the use of hard fault diagnostic procedures when intermittent faults are occurring.